In a pulp mill where paper is made, wood chips processed from debarked logs are cooked in a soda solution in a high-pressure vessel known as a digester. The soda solution at high temperature and pressure dissolves resins (lignin) binding cellulose fibers in the wood chips. The cellulose fibers are separated, washed, bleached and further processed to manufacture paper, or for other applications. After cellulose fiber separation, what is left is an aqueous solution. The aqueous solution is concentrated by evaporation to a concentration of approximately one-third water. The rest is combustible resin (lignin) and chemicals, which can be recovered. Up to 98% of the chemicals used in the process can be recovered. Moreover, the resins constitute an excellent fuel. Universal practice is to thus concentrate the solution by evaporation and make various chemical adjustments to form what is known as black liquor, and then to burn the black liquor as fuel in a recovery boiler.
A recovery boiler is a large structure; perhaps fifteen stories high, and thirty to forty feet (9.1 to 12.2 meters) wide. The lower portion of a recovery boiler where combustion occurs is known as the furnace, and is the hottest part. The walls of a recovery boiler are walls formed of water-carrying tubes that are heated by the combustion process to usefully generate steam. Within the recovery boiler, the resins constituting part of the black liquor are burned to produce heat and waste gases. Chemicals in the black liquor, such as soda, form a molten residue known as smelt, which is recovered. A complicating factor in this process is that the smelt temperature is approximately 2000° F. (1093° C.) to 2100° F. (1149° C.). The smelt and gases within the recovery boiler are chemically highly active at these temperatures. Also, during boiler operation, the black liquor is typically sprayed from a number of nozzles directly against the walls of the lower portion of the boiler. Thus, the carbon steel water-carrying tubes are subject to corrosion and eventual destruction, which necessitates replacement of the boiler wall. Moreover, failure of the water-carrying tubes is potentially catastrophic as an explosion can occur if water within the tubes comes into with the hot smelt which can be around, and sometimes is above, 2000° F.
A common practice in recovery boilers, particularly in the furnace portion, is to employ a multiplicity of cylindrical studs, analogous to heat-exchange fins, for corrosion protection. Each stud has a base or attachment end welded to the external surface of a water-carrying tube, and an exposed or tip end projecting radially outward from the tube. Conventional studs are made of low carbon steel and, when new, are typically ⅜ inch (0.95 cm) or ½ inch (1.27 cm) in diameter, and ¾ inch (1.91 cm) in length. The stud may have a sleeve of a different material as disclosed in my U.S. Pat. No. 5,107,798. Studs typically are applied at a uniform density of sixty or ninety studs per lineal foot (30.5 cm) of 2½ to 3-inch (6.5 to 7.62 cm) diameter water-carrying tube. The distance between adjacent rows of studs is called pitch while the distance between adjacent studs within a row is called spacing. A common pitch in the industry is one-fourth inch measured from the centers of the studs. Pitch is often somewhat larger than spacing, typically five-eights of an inch, but could be the same distance as the spacing. A recovery boiler may have anywhere from 100,000 to 1,000,000 studs in total.
It is standard practice in the industry to periodically measure the thickness of the walls of the tubes so that replacement can be made before the walls have corroded to such an extend that they rupture. Measurements are made with an ultrasonic probe using techniques similar to those described in U.S. Pat. Nos. 4,685,334 to Latimer and U.S. Pat. No. 4,669,310 to Lester. Before any measurements are made, the slag that has built up on the wall is removed. Since the slag in a pulp boiler is water soluble, simple washing removes the slag. Next, the portion of the tube to be tested is blasted to remove the oxide coating. Then a conductive gel is applied to the test location and the probe is placed against the gel on the wall of the tube. Ultrasonic waves are directed from the probe through the conductive gel into the tube wall and reflected waves are detected. The response time of the reflected waves is then used to calculate the thickness of the tube wall. The face of a typical probe used for such tests is one half inch or about one centimeter in diameter. Smaller diameter probes of about one-fourth inch in diameter could be used, but they tend to lose contact with the surface more easily than the half-inch diameter probes. Consequently, the larger, half-inch diameter probes are preferred. Several locations on the furnace wall are tested in a single test session.
Those who conduct these periodic tests strive to take their measurements at the same locations on the boiler wall for each successive test. Since the tubes and studs are subject to corrosion and erosion it is not possible to mark the location of a measurement made at one point in time and to have that mark be present when the next measurement is taken several weeks or months later. Consequently, the technicians conducting the tests take measurements from the base of the boiler wall or other reference point when the first test is conducted and try to use those measurements to find the same location when subsequent tests are made. The measurement process is time consuming and subject to error.
Repeated test measurements taken at the same location over time can be used to calculate a rate of corrosion. Knowing the corrosion rate enables the boiler operator to predict when tube failure will occur. Then the tube can be replaced before it fails. Most, if not all, boilers are shut down periodically for inspection and maintenance. The shutdowns are scheduled months in advance so as to have a minimum impact on the operation of the plant. Thus, the best time to replace a boiler tube is during a scheduled shutdown. But, the boiler owner does not want to replace tubes until they are close to the end of their useful life. The calculation of corrosion rates based upon repeated tests rests upon the assumption that successive measurements are taken at the same location. If measurements are taken in different locations they may lead to an erroneously determined corrosion rate. Yet, under the current practice, successive corrosion measurements are seldom made at precisely the same location.
There is a need for a studded boiler wall that is configured to enable easy corrosion testing while assuring that successive measurements are taken in precisely the same location.